a special anniversary (or the celebration of it)
"The town organized a grand parade to celebrate their diamond jubilee, marking fifty years since they first incorporated as an official city."
A special year of emancipation supposed to be kept every fifty years, when farming was abandoned and Hebrew slaves were set free.
"The ancient scrolls described how the land would lie fallow during the jubilee, allowing it to recover after four decades of cultivation."
a year of rest, observed by the Israelites every 50 years